Communication system



Sept. 30; 1941. R. c. MATHES 2,257,568

COMMUNICATION SYSTEM Filed June 14, 1939 '7 Sheets-Sheet 1 FIG 3 FIG 6 F/G.4 FIG. 7

'FIG. 8

FIG. F1612 FIGS lNl/ENTOR BY R6. MATHES WWWW ATTORNEY Sept. 30, 1941. r R. c. MATHES 2,257,563

COMMUNICATION SYSTEM Filed June 14, 1939 7 Sheets-Sheet 2 PHASE SHIFTER INVENTOR R. C MATHES WWW ATTORNEY Sept. 30, 1941. R. c. MATHES 7, 8

COMMUNICATION SYSTEM Filed June 14, 1959 7 Sheets-Sheet 3 lNl/ENTOR R. C. MATHES ATrnRA/Ef P 1941- R. c. MATHES 2,257,568

COMMUNICATION SYSTEM Filed June 14, 1939 '7 Sheets-Sheet 4 INVENTOR By R. G. MATHES FIG. 4

P 1941- R. c. MATHES 2.257568 COMMUNICATION SYSTEM Filed June 14, 1959 7 Sheets-Sheet 5 POLARIZED l Mm FELTER INVENTOR W R. C. MATHES ATTORNEY Se t. 30', 1941. C MATHEs 2,257,568

COMMUNICATION SYSTEM.

FiledJune 14, 1959 7 Sheets-Sheet'fi FIG 6 lg I INVENTOR BVR. c. MA 71155 W W 1 (C ATTORNEY Sept. 30, 1941. R. c. MATHES 2,257,568

COMMUNICATION SYSTEM '7 Sheets-Sheet]? Filed June 14, 1939.

INVENTOR By R. C. MAT/1E5 WWWCW ATTORNEY Patented Sept. 30, 1941 UNITED STATES PATENT OFFICE COMMUNICATION SYSTEM Robert C. Mathes, Maplewood, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 14, 1939, Serial No. 279,015

12 Claims. (Cl. 17916) This invention relates to communication systems and particularly to those in which connections are established automatically.

The objects of the invention are to obtain greater efliciency and increased speed in the transmission of wanted designations, such as telephone numbers; to simplify the operations necessary on the part of the subscribers and others; to reduce the holding time of the equipment at the central stations and exchanges; and otherwise to secure improvements and advantages in systems of this kind.

Systems have been proposed heretofore for transmitting telephone designations, such as the office names and numbers of subscribers lines or the numbers of trunk circuits, from one point to another by means of currents of different frequencies, the frequencies singly or in combination serving to identify the different numerals or other characters of the designation. It has been necessary, however, with this method of signaling to provide frequency selective devices at the central oilice, such as selective filters or mechanically tuned elements, to discriminate between the currents of different frequencies.

According to a feature of the present inven-' tion advantages over these prior systems are realized by means of a system in which current of a single frequency is used for transmitting designations and in which the varying values of the digital characters of the designations are obtained by producing variations in the phase of the current. For example, the subscribers substation on a telephone line is equipped with means for shifting the phase of an alternating current to characterize the successive letters or numbers in a telephone designation. At the central office means is provided for determining the amount of each shift of phase and to identify thereby the successive characters of the designation. The identified characters are then registered on relays or other suitable registering means and utilized for the purpose of selectively controlling the operation of automatic switches to establish the desired connection.

Another feature of the invention is an arrangement in which two alternating currents of the same frequency are produced, one in the subscribers line and one at the central office, in

which the current in the subscribers line, after having its phase slnfted by a given amount according to the setting of the subscribers transmitter to characterize a designation, is transmitted to the central ofiice, and in which means at the central oilice compares the two currents and measures the phase difference to identify the value of the digital character represented by the current of shifted phase received from the subscribers line.

Another feature of the invention is a system of the foregoing character in which a preliminary adjustment is automaticall made in the phase of the current produced at th central office to compensate for the shift produced in the phase of the current, flowing in the calling line, as a result of the inherent characteristics of said line. This arrangement takes into account the varying characteristics of the different subscribers lines and makes it possible to obtain a uniform measure of the phase shift produced by any subscribers designation transmitter unaffected by the shift resulting from the subscribers line.

Another feature of the invention'is a system in which the subscriber is able to preset his transmitter for the entire designation before any signals are sent to the central office, in which the currents of shifted phase representing the oflice name are first sent and registered inthe originating oilice, in which a pause is then introduced until the switches in the originating oflice are set to extend the connection, either to the terminating equipment in the same ofiice or to the-terminating equipment in a distant oflice, according to the office designation received, and in'which the numerical part of the designation preset on the subscribers transmitter is then sent over the line and the extended connection to the called oiiice where it is similarly identified and registered andused to control the selective switches therein to. complete the desired connection.

These and other features of the invention will be described more fully in the following specification and will also be set forth in the appended claims.

The several figures of the drawings, when arranged as indicated in- Fig. 8, illustrate a portion of an automatic telephone system embodying the invention.

Considering the several figures in particular:

Fig. 1 shows a subscribers line equipped at the substation with a preset transmitter for controlling the phase of the signaling current;

Fig.2 at the top illustrates diagrammatically the line finder and selector. switches comprising the originating switching equipment in the central oiiice to which the subscribers line belongs;

The remainder of Fig. Zand Figs. 3 and 4 taken together disclose one of the originating register senders which receive the offlce code part of the designations transmitted from the calling lines for controlling the switches shown in Fig. 2;

Fig. 5 at the top illustrates diagrammatically the switches associated with the terminating equipment in a central office; and

The remainder of Fig. 5 and Figs. 6 and '1 taken together show one of the terminating register senders which receive the numerical part of the designations from the calling lines for controlling the selector switches shown in Fig. 5.

The invention is applicable particularly to systerns in which the digital information for a call is registered in a common register sender in the central ofiice, the registered information being utilized to control the switches in the central office to establish the desired telephone connection. To this end the system chosen to illustrate the invention is one in which the central offices are equipped with automatic switches of the well-known panel type. It will be understood, however, that other forms of switches, such as step-by-step switches and cross-bar switches, could be used to establish the connections. In the system shown in the drawings many details have been omitted, such as the structure and circuits of the selective switches, the manner in which the sender selectors seize idle senders in response to incoming calls, the manner in which the registers function to control the operation of the selector switches, and numerous other details of operation. For a better understanding of these details reference may be had to many patents in the prior art relating to systems of this general character and in particular to the following patents: Stearn et al. No. 1,395,977 of November 1, 1921; Steam No. 1,505,171 of August 19, 1924; and Raymond et al. No; 1,862,549 of June 14, 1932.

General description Referring to the drawings, the subscribers line I is provided with the usual microphone IOI, receiver I02, ringer I03, induction coil I04 and switchhook I05.

The substation is also provided with a preset designation transmitter I by means of which the subscriber may set up the designation before any of the signals are transmitted to the central oflice. The designation transmitter comprises a plurality of manually adjustable elements, one for each of the successive characters in a telephone designation. In the transmitter disclose-d there are seven of these adjustable elements, assuming that the office names are expressed by the first three digits whi-chmay be letters or letters and numerals in combination, and that the numbers of the lines themselves are expressed in four digits. The first three or office code elements consist of manually rotatable shafts I01, I08 and I09, for selecting electrical terminals. The shaft I01 carries the two brushes or wipers H0 and III, each of which engages a series of ten terminals. Likewise the shaft I08 is provided with wipers H2 and H3 and the shaft I09 with wipers H4 and H5. The shafts I01, I08 and I09 are rotatable by the subscriber by means of knob handles H6, H1 and I I8, respectively. The four shafts and wiper sets corresponding to the digits of the line number are similar to the three already described. Only two of these numerical designation elements are shown, namely, the one corresponding to the thousands digit, comprising shaft H9 and associated wipers I20 and I'll, and

the one corresponding to the units digit comprising the shaft I22 and associated wipers I23 and I24. These shafts are rotatable in a similar manner by means of the handles I25 and I25, respectively.

.The several contact banks of the transmitter are connected to electrical networks comprising a series of condensers of different capacities, resistance elements, and reactance coils. The purpose of these networks is to shift the phase of an alternating current flowing in the subscribers line I00 by varying amounts according to the setting of the digital shafts I01 and I08, etc. The alternating current which is applied to these shifting networks is derived from a current of fundamental frequency transmitted from the central oflice over the subscribers line. This fundamental frequency current is applied to the harmonic generator I21 at the substation, and a suitable harmonic thereof, say the second one, is selected by filter I28 and applied to the phase shifter networks. After undergoing the phase shift the harmonic current is impressed on the line I00 and transmitted back to the central office for phase comparison. The means by which the current with its shifted phase is applied to the line for transmission to the central ofiice comprises an escapement-driven switch I29 including a rotary shaft I30 and associated wipers I3I and I32 and an escapement mechanism I33. This escapem-ent switch I29 is set by the subscriber, after he has preset the transmitter, and is stepped automatically under control of the central ofiice to cause the successive transmission over the line of the phase-shifted currents corresponding to the successive digital characters of the designation.

The subscribers line I00 terminates at the central office in the bank of line-finder switch F. Finder F is individual to the first or district selector switch S-I, which has access to an office selector switch S2, which in turn has access to trunks, such as the trunk 200, extending to the selector switches comprising the terminating switching equipment either in the same or in a distant central office. The link circuit comprising the line finder F and the district selector S-I is provided with a sender selector switch S-3 for seizing an idle originating sender, such as the one shown in Figs. 2, 3 and 4.

The originating sender includes a source of alternating current 201 of fundamental frequency, such as frequency f1. Current from the source 20I is sent over the subscribers line to the substation for producing the second harmonic fre quency f2, as explained. Also current of frequency f1 is applied from the source 20l to a local harmonic generator 202 for producing in the central ofiice a harmonic current of frequency f2. Since these two harmonic currents, the one produced by the harmonic generator I21 at the substation and the one produced by the harmonic generator 202 at the central oflice, are derived from the same fundamental source 20 I it is possible, by bringing them together in the central office and there comparing their phases, to determine the particular phase-shifting network that the subscriber at the substation has inserted in the circuit when he set his designation transmitter. This phase comparison is made at the central ofiice by applying the phase-shifted harmonic returned from the subscribers station and the harmonic produced in the central oflice to a balancing circuit, deliberately shifting the phase 9i the locally produced harmonic until a balance is attained, and measuring the amount of shift required to produce the balance- The'amount of shift required to produce the balance identifies the particular shifting network inserted by the.

subscriber and is, therefore, an identification of the digital character set up. on his. transmitter. The mechanism for making thisphase comparison for each of the successive digits of a designation is illustrated inFig. 3' and will be explained presently. Before doing so, however; an explanation will be given of the compensating shift that is made in the phase of the locally produced harmonic to account for the inherent phase-shifting property of the particular calling line concerned.

The compensating phase shift is likewise performed by a balancing circuit and operates in conjunction with the phase-shifting device 203. The balancing circuit for the compensating shift is shown in Fig. 2 and comprises vacuum tubes 204 and 205, polarized relay 206. gas-filled tubes 207 and 208, relays 2'09 and 2I0 and associated circuits. By applying the harmonic received from the subscribers station, when subjected only to the phase-shifting characteristics of the subscribers line I00, and the unshifted harmonic produced by the harmonic generator 202 of the central oflice to the input circuit of the balancing mechanism, the phase difference between these two harmonics unbalances the circuit and causes the motor ZII to rotate the phase shifter 203' in the proper direction and a sufiicient amount to bring the circuit into balance. Thus the phase of the local harmonic is shifted preliminarily by an amount just sufiicient to compensate for the inherent phase shift imposed by the subscribers line upon the harmonic produced at the subscribers station.

Thereafter the successive digits of the preset designation are transmitted and identified by applying the locally produced harmonic and the incoming phase-shifted harmonics to the balancing circuit shown in Fig. 3. This circuit includes a stepping switch 300 having wipers 301, 302 and 303, vacuum tubes 304 and 305'and relays 305 and 301. The terminal banks with which wipers 3M and 302 cooperate are connected to condensers having different capacities. These condensers, and the reactances 308 and 309, and resistances 343 and. 344 form ten different electrical networks, each of which corresponds to one of the ten values of a digital'character. When the incoming phase-shifted harmonic from the subscribers line is applied to the input circuit of the tubes 304 and 305 and the-locally produced harmonic is applied tothe input circuit of these tubes simultaneously through the phase-shifting networks associated with switch 300, the resulting unbalance causes the switch 300 to rotate, automatically changing the network successively and, consequently, shifting the phase of the local harmonic correspondingly, until the two harmonics are brought into a phase balancing relation. Thereupon the switch 300 stops, and the position of itswiper 303 identifies the value of the corresponding digital character. The bank of wiper 3031s wired to a chain of digital relays 400, and a relay in this chain and a corresponding relay in the chain 40I operate to identify the particular digit. Following this the switch 300 is restored to'normal, and the same operation is performed repeatedly for the succeeding digits of the designation set up at the substation. During these balancing operations, the several digit indicating elements of the subscribers transmitter are brought successively into circuit relation with i dreds the phase balancing mechanism in the central ofiice. Thisv is accomplished by sending an im pulse of alternating. current from the source 2I2 over the. line upon the completion of each balancing operation. These. signals operate the esoapement switch I29, onev step at a time, to connect the successive digital elements and their associated networks to the line. Each of the successive digits indicated on the chain relays 400 and 40I is transferred immediately and permanently registered on the registers. There are three of these registers 402, 403 and 404 for the corresponding characters of the ofiice designation. The register relays once operated lock and later serve to control the selective operation of the switches S-I- and 8-2 to extend the calling line over a trunk 200 to the terminating equipment in the desired oflice.

The terminating equipment includes an incomi-ng selector switch S4 and a final selector switch S-5. dividual to theincoming trunk 200 serves to corn nect said trunk to an idle terminating register sender, such as the one shown in Figs. 5, 6 and 'l.

The terminating register sender is substantially the same as the originating sender shown in Figs. 2, 3 and 4. It includes a source of alternating current 500- of the fundamental frequency f1. Current from this source is supplied over the trunk 200 and thence over line I00 to the subscribers station where it is utilized to produce the harmonic frequency f; for transmitting the four numerical characters of the designation. Current from the source 500 is also applied to the harmonic generator I to produce a local harmonic of frequency f2 for phase comparison with the harmonic of frequency fzreturned from the subscribers line. The preliminary adjustment of the phase of the local harmonic f2, to compensate for the phase-shifting properties of the subscribers line I00 and trunk 200, is accomplished by the motor-driven phase shifter 502 and a phase balancing circuit including vacuum tubes 503 and 504, gas tubes 505 and 506- and associated relays. The incoming sender also includes a stepping switch 600 and associated phase-shifting networks for adjusting the phase of the locally produced harmonic f2 to balance with the phase of the harmonic incoming from the subscriber's line. The balancing circuit includes the vacuum tubes GUI and 602 and the associated relays. For each of the balancing operations, corresponding respectively to the thousands, hundreds, tens and units digits, the incoming sender transmits an impulse of alternating current from the source 501, of frequency is, back to the subscribers station to produce the necessary stepping operation of the escapement switch I29. The incoming sender also includes the chain relays I00 and IM and numerical relay registers I02, I03 and I04. Although only three registers are shown, there are in fact four of these relay registers. Register I02 receives the thousands digits of the subscriber's number, register 103 receives the hundigits, an intermediate register (not shown) receives the tens digit, and register I04 receives the units digit.

A limited number of filters are disclosed diagrammatically both at the subscribers station and in the central offioe. The purpose of these filters is to discriminate between the currents of different frequencies (f1, f2 and fa) utilized in the operation of the system. These filters may be of any suitable and Well-known design. Also the harmonic generators I 21, 202 and 50I are illus- A sender selector switch S-t in-.

trated diagrammatically since these devices are well-known in the art. If desirable these generators may be of the type comprising non-linear rectifier elements, or they may be of the type making use of a saturable reactance element. A generator of the latter type is disclosed in Patent No. 2,164,335, issued July 4, 1939. The phase shifters 203 and 502 are likewise illustrated in diagrammatic manner, these devices being old and well known in the art. The details of one type of shifter suitable for this purpose are disclosed in the patent to Meacham, No. 2,004,613 of Junell, 1935.

Detailed description The system will now be described in detail, and for this purpose it will be assumed that the subscriber of line I wishes to converse with the subscriber of line 501. Assume further that line 501 terminates in a central oflice having the code designation CHE and the numerical designation 5678. Since the code letters C, H, and E correspond to the numerical designations 2, 4 and 3 in dialing systems using decimal impulses, it will be assumed that these letters have the same numerical values in the present system. The calling subscriber, having first determined the desi nation of the called line, rotates the shafts I01, I08 and I09 to position the associated wipers on the second, fourth and third position terminals, respectively. Although not illustrated, the subscribers transmitter would be provided with indicating means showing the positions of the various code letters and numerals to assist him in setting the contact wipers on the proper terminals. That is to say for the letter C he would rotate the shaft I01 to bring the wipers H0 and III in engagement with terminals I34 and I35; he would rotate the shaft I09 to set the wipers H2 and H3 on terminals I39 and I31; and he would rotate the shaft I09 to set the wipers H4 and H5 on terminals I38 and I30, respectively. Following this he would similarly rotate the four shafts representing the numerical digits to set their wipers on the fifth, sixth, seventh and eighth position terminals, respectively. Having positioned the transmitter in accordance with the full designation, the subscriber now seizes the knob I40, and rotates the shaft I30 of the escapement switch I20 to move the wipers I3! and I32 from their normal positions to their operated positions, at the same time tensioning the driving spring I54. In the set or operated position of the escapement switch I29 the wipers I3I and I32 rest on terminals MI and I42, respectively. The escapement mechanism I33 holds the shaft in its operated position against the tension of spring I54.

The subscribers transmitter is now positioned ready to cooperate with the equipment at the central office to, transfer the designation thereto. To effect this transfer operation the subscriber removes his receiver I02 from the switchhook and throws the key I43 to its operated position where it looks and remains locked until he restores it to normal. Operating the key I43 disconnects the subscribers line I00 from the substation instrument and connects the line to the primary winding of the transformer I44 associated with the designation transmitter. This effects a closure of a bridge across the tip and ring conductors of the line I00 to cause the operation of the line relay at the central ofiice. The bridging circuit may be traced from the upper conductor of line I00 through the closed transformer 233.

switchhook contacts, left operated contacts of key I43, conductor I45 through the primary winding of the transformer I44, conductor I46, through the right closed contacts of key I43 to the other side of the line I00. The operation of the line relay (not shown) causes the finder F at the central office to find the calling line I00. When the line is found, sender selector switch S-3 operates to hunt for an idle originating register sender. When the selector S-3 seizes an idle sender, which is assumed to be the one shown in Figs. 2, 3 and 4, a circuit i closed from battery through the relay 2I3 in the sender, conductor 225, brush 2I4, closed contacts of the spring 2I5 of the controlling sequence switch associated with the line finder-district selector link, through the brush 2l6 of finder switch F, thence over the subscribers line I00 through the primary winding of transformer I44 and returning over the other side of the line, brush 2 I1, sequence switch spring 2I8, selector brush 2I9, conductor 225, reactance coil 220 to ground. Relay 2 I 3 operates and closes an obvious operating circuit for slow-release relay 22I. Relay 2I3 also closes a circuit for relay 222, but this latter relay, being slow to operate, does not attract its armatures until the balancing circuit has had an opportunity to become effective as will be explained presently.

At the same time that the operating circuit is completed for the relay 2I3, a circuit is closed for supplying current from the source 20I to the calling subscriber's line. This circuit may be traced from the right pole of the source 20 I, conductor 223, through the filter 224, conductor 225, thence over the line through the primary of transformer I44 and returning over conductor 226 through the filter 224 and conductor 221 to the other pole of the source 20I. Current of frequency I1 is induced in the secondary winding of the transformer I44 from whence it passes through the selective filter I41 to the harmonic generator I21. Current of the frequency f2 produced by the generator I21 is selected by filter I28 and is applied to the line I00 to be returned to the central office. Since the escapement switch I29 is standing on contacts MI and I42, a circuit is provided for applying the harmonic current to the line I00 free from the influence of the phase-shifting networks associated with the transmitter I09. This circuit may be traced from the upper left pole of the harmonic generator I21 through the filter I29, conductor I48, conductor I49, terminal I42 and wiper I32, conductor I50, through the secondary winding of the transformer I44, thence over conductors I5I and I52, terminal HI and wiper I3I, conductor I53, through the filter I29 to the lower left pole of harmonic generator I21. Therefore, current of the frequency I2 is induced in the primary winding of transformer I44 and flows back over the line I00 to the originating sender. Here it is applied by way of conductors 225 and 223 to the selective filter 220. After passing the filter 228, the harmonic current is amplified by the amplifier 229 and is then applied through the back contacts of relay 230, to the primary winding of transformer 23I in the input circuit of the tubes 204 and 205.

It will be noted that current from the same fundamental source 20I is applied to the harmonic generator 202, and the second harmonic (frequency i2) is selected by filter 232 and applied by way of phase shifter 203 and back contacts of relay 230 to the primary Winding of This local harmonic, which is iaastgse constantly applied to the input circuit of the tubes 204 and 205, while the sender is in its normal condition, affects both tubes similarly with the result that the polarized relay 206 in the output circuit of the tubes maintains its neutral position. As soon, however, as the incoming harmonic from the subscribers line is applied to the input circuit of tubes 204 and 205 by way of transformer 23!, any difference in phase, greater or less than 90 degrees, that may exist between the local harmonic and the incoming harmonic by reason of the inherent phase-shifting characteristics of the subscriber's line I will unbalance the tubes 204 and 205 causing relay 206 to throw its armature to one or the other of its contacts as the case may be. This effect is obtained by adjusting the bias so that the tubes operate on the non-linear portion of their characteristics.

It will also be noted that the gas-filled tubes 20'! and 203 normally have their control gaps ionized. The ionizing circuit for tube 201 may be traced from the positive pole of battery through resistance 234, control electrode 235, cathode 236 to the opposite pole of the battery. A similar circuit for tube 208 may be traced from the positive pole of battery, resistance 231, control electrode 238, cathode 239 to the opposite pole of battery. The main gaps of these tubes, however, are opened at the contacts of slow-operating relay 222. As soon as the polarized relay 200 throws its armatures to one of its contacts when the sender is seized and the tubes 204 and 205 unbalanced as above described, a circuit is closed for quenching the control gap of the corresponding gas-filled tube. Assume that the phase unbalance causes the relay 205 to close its right contacts. This closes a short circuit around the control gap of tube 208, and this tube deionizes. When the slow-operate relay 222 closes its contacts, which does not occur until after relay 206 has had time to operate and deionize tube 208, the anode circuit is closed for the tube 201. This circuit may be traced from positive pole of battery 240, Winding of relay 209, anode 24 I, cathode 236, contacts of relay 222 to the negative pole of battery 240. The control gap of tube 201 being in an ionized condition, the main gap ionizes and relay 209 operates.- Had the tubes 204 and 205 been unbalanced in the opposite sense, relay 206 would have operated to its left contact, quenching the tube 201. This Would have resulted in the firing of the main gap of tube 208 and the operation of relay 2 I 0 instead of relay 209. When. relay 209 operates, it causes the operation of relay 242 whereas the operation of relay 2|0 causes the operation of relay 243. Relay 242 starts the driving motor 2 to operate the phase shifter 203 in such a direction as to bring the phase of the locally produced harmonic into quadrature with the phase of the harmonic incoming from the subscribers line. In a similar manner relay 243 starts the motor in the opposite direction for the same purpose when the tubes are unbalanced in the opposite sense. Assuming relay 242 is the one operated, a circuit is closed as follows for exciting the field 245 of the motor: Positive pole of battery through the contact of relay 244, through the field 245 of the motor to the opposite pole of battery. Relay 244 operated, when relay 222 energized, in a circuit through the contacts of relay 222, conductor 246 to ground over a circuit leading through either a contact ofrelay 209 or a contact of relay 210. Also the armature circuit of the motor is closed from the positive pole of battery through the contacts of relay 244, inner contacts of relay 242 through the armature of the motor, outer front contacts of relay 242 to the opposite pole of battery. By inspection it will be found that relay 243 closes a circuit for the flow of current through the armature of the motor in the opposite direction.

The motor 2 drives the phase shifter 203 until the phase of the local harmonic applied to the transformer 233 is in quadrature with the phase of the harmonic incoming from the subscribers line and applied to the transformer 23 l. -When this quadrature relation is obtained, the tubes 204 and 205 balance, relay 206 assumes its neutral position, and the control gap of the quenched tube 208 reionizes. This immediately ion-izes the main gap of the tube 208, and relay 2|0 operates. Since both relays 209 and 210 are now operated, relay 242 releases and opens the energizing circuit of the armature of the motor. With both relays 242 and 243 released, an obvious short circuit is closed across the armature of the motor, and, since slow-release relay .244 holds the field winding energized for a moment, the short circuit on the armature produces a dynamic braking action to stop the phase shifter accurately in its adjusted position. A moment later relay 244 releases and opens the field circuit of the motor. The phase of the locally produced harmonic has now been adjusted to compensate for the phase shifting of the calling subscribers line H30 and may be used to measure the successive shifts produced by the different networks inserted at the subscribers transmitter for the purpose of transmitting the several characters of the designation.

When the second one of the two'relays 209 and 2E0 operates as the compensating balance is reached, an obvious circuit is closed-for the energization of relay 230. Relay 230 closes an energizing circuit for slow-to-operate relay 310. Relay 3l0, however, does not attract its armatures for an appreciable interval of time during which certain circuit operations, about to be described, are performed. Relay 230 at its innermost right contacts disconnects the output circuit of the phase shifter 203 from the transformer 233 and extends this circuit to the primary winding of transformer 3 through the phase-shifting networks associated with the input circuit of the balancing tubes 304 and 305. The locally produced harmonic of frequency is is thus supplied to the input circuit of the tubes 304 and 305, the circuit therefor extending from phase shifter 203 over conductor 247, through the right front contact of relay 230, conductor 248, reactance coil 309, through the primary winding of transformer 3| I, reactance coil 308, conductor 249, innermost right front contact of relay 230 to conductor 250. The wipers 30! and 302 of the switch 300 serve in successive positions to connect diiferent ones of the condensers in parallel with the coils 308 and 309, respectively. These coils in combinationwith resistances 343 and 344 and with different ones of the condensers produce different amounts of shift in the phase of the harmonic f2 flowing in the'primary winding of the input transformer 3. At the same time relay 230 at its outermost right contacts disconnects the output circuit of amplifier 229 from the transformer 23! and extends this circuit through to the primary winding of transformer 3l'2. Thus the harmonic f2 incoming from the sub-scribers station is applied to the input transformer 3| 2 simultaneously with the application of the local .90 degrees.

harmonic to the transformer 3I I. The tubes 304 and 305, which are normally balanced to main tain the armature of the differential relay 306 in its released position, will become unbalanced when these two harmonics are applied to the input circuits provided the phase difference between said harmonics is something other than This result is obtained by a bias which causes the tubes to operate on the nonlinear part of their characteristics.

Since the wipers I3I and I32 of the escapement switch I29 are standing on their contacts I4I and I42, respectively, excluding the phaseshifting networks of the subscribers transmitter, the phase shift in the harmonic f2 transmitted from the subscribers line to the transformer 3I2 is that which results only from the phase-shifting characteristics of the line. Since the wipers 301 and 302 of the phase-shifting switch 300 are assumed to be standing in their normal positions,

the phase of the local harmonic f2 applied to the transformer 3I I is likewise shifted by an amount corresponding to the phase displacement produced by the subscribers line. When, therefore, these two harmonics are applied to the balancing circuit including tubes 304 and 305, this circuit maintains its balanced condition, and relay 306 remains deenergized. At the time relay 230 operates, a circuit is closed as follows and remains closed until relay 3 I operates shortly thereafter: From the lower pole of source 2 I2 over conductor 25!, inner left contact of relay 3I0, outer left contact of relay 230, conductor 252, conductor 225, thence over the subscribers line through transformer I44 and returning to conductor 226 and the other pole of the source 2I2. Current from the source 2 I2 is induced in the secondary winding of transformer I44 at the substation, is selectively passed by the filter I55, is rectified by the full wave rectifier I56, and is applied to the escapement magnet I51. Magnet I51 energizes preparatory to advancing the wipers I3I and I32 to the next set of terminals. When, following the operation of relay 230, relay 3H] attracts its armatures, it opens the circuit of the source 212, permitting the magnet I51 at the substation to release. The escapement I33 now permits the wipers I3I and I32 to complete their advance into engagement with the next terminals I58 and I59, respectively. The escapement switch I29 in this position includes the network, comprising reactance coils I60 and I6I, resistances I12 and I13, and condensers I62 and I63, corresponding to the first office code character C. The circuit, including the phase-shifting network, over which the current of harmonic frequency f2 flows from the filter I28 to the transformer I44 may be traced from conductor I53, reactance coil I60, conductors I52 and II through the lower winding of transformer I44, conductor I50, reactance coil I61, conductor I49 and conductor I48 returning to the filter I23. It will also be seen that the resistances I12 and I13 of the networks are included in a series circuit with the transformer winding. The wiper I32 connects the condenser I63 in parallel with coil I6I over a circuit traceable from the upper terminal of said coil, wiper I32, switch terminal I58, wiper II I and terminal I35, condenser I03, thence over conductor I49 to the lower terminal of coil I6I. Similarly Wiper I3I connects condenser I62 in parallel with reactance coil I60. Since the character C is the equivalent of the numeral 2, the phase of the harmonic f2 transmitted over the line to the transformer'3l2 is shifted by the substation transmitter network by an amount characterizing the digital numeral 2. Consequently the phases of the two harmonics now applied to the input of the tubes 304 and 305, one representing the numeral 2' and the other of which rep-resents the compensation made for the subscribers line, differ from each other such that the tubes become unbalanced, and relay 306 operates. Relay 306 opens the circuit of relay 301, and the latter relay, if previously operated, nowreleases.

Relay 3I0 in operating as above described renders effective the relaxation oscillator 3I1, comprising discharge tube 3I8, relay 3I9, condenser 320, and resistance 32I, to step the switch 300 from terminal to terminal until the tubes 304 and 305 are balanced by bringing the phase of the locally produced harmonic into quadrature with the phase of the harmonic incoming from the subscribers line. The oscillator 3I1 is made effective by closing the charging circuit for condenser 320. This charging circuit may be traced from the positive pole of battery 322, conductor 341, through the outer right contacts of relay 3I0, inner'right normal contacts of relay 323, conductor 324,v outer normal contacts of relay 301, conductor 325, through normal series contacts of the relays of register 404, conductor 405,

resistance 32I to one pole of condenser 320, the

other pole of the condenser being connected to the negative pole of battery 322. Condenser 320 charges at a rate depending upon the value of resistance 32I until it reaches the ionizing potential of the tube 3I8. Thereupon the tube ionizes, and current flows through the winding of relay 3 I9 to operate the same, and the condenser 320 discharges. Relay 3I9 energizes the stepping magnet 326' of switch 300, which attracts its armature preparatory to advancing the wipers to the first position terminals. When the condenser 320 has discharged sufliciently, tube 3I0 deionizesyand relay 3I9 releases to open the circuit of the stepping magnet, The magnet thereupon releases its armature, and the wipers 30I, 302

and 303 are advanced to the first position terminals in the bank including a pair of condensers 3I5 and 3IE in the network to change the phase of the locally produced harmonic f2. The condenser 320 recharges and fires the tube, causing the switch 300 to take another step. This process continues, the switch 300 including successive pairs of condensers in the phase-shifting network' until the phase of the local harmonic I2 is brought into quadrature relation with the phase of the incoming harmonic from the subscribers line. When this occurs, the tubes 304 and 305 balance, and relay 306 releases since equal currents are flowing through its differential windings. Relay 306 in releasing closes a circuit from ground through its normal contacts, conductor 321, closed contacts of relay 3 I 0 through the winding of relay 301 to battery. Relay 301 at its outer contacts opens the charging circuit of condenser 320 to disable the oscillator 3I1 and prevent further stepping of the switch 300. At its inner contacts relay 301 closes a circuit from ground over conductor 328, closed contacts of relay 3I0, conductor 329, normal contacts of relay 323, conductor 330 to the wiper 303 of switch 300. Inasmuch as the movement of the switch 300 in producing a phase balance between the two harmonics of frequency, f2 is a measure of the digital character selected by the subscriber, the position of the wiper 303 in its bank of ten terminals is a direct indication of the numeral value of the character. Under the assumption that the numeral value of the letter C is 2, the switch300 takes two steps in the first balancing operation, and brush 303 comes to rest on terminal 331. Therefore, the circuit above traced from the grounded contact of relay 301 to the wiper 303 is further extended over terminal 331, conductor'332, through the winding of relay 406 of the chain 400 to battery over conductor 40'1. Relay 406 operates and closes an obvious energizing circuit for the corresponding relay 408 of the chain 401.. Relay 406 also closes a circuit from battery through the winding of relay 323, conductor 333, left contact of relay 406 to ground. Relay 323, however, is slow to operate and does not attract its armatures until an interval has expired during which certain circuit operations, about to be described, are performed.

The purpose of the chain relays 400 and 401 is to take a temporary registration of the successive digits detected by the switch 300 and to transfer them to the permanent registers 402, 403 and 404. To this end the chain relays 401 are associated successively with the registers 402, 403 and 464 by the connecting relays 409, 410 and 41 I. The first of the connecting relays, relay 409, is already operated, having energized over a circuit from battery through its winding, conductor 412, outer contacts of relay 221, conductor .253 to ground at the normal left contact of relay 436. At the time chain relays 406 and 408 operate as above explained a circuit .is closed as follows for the operation of relay 254: Battery through the winding of relay 254, conductor 420, inner right front contact of relay '406 to ground over conductor 419. Relay 254 operates and at its front contacts closes a circuit from ground through the normal contacts of relay 255, over conductor 256 through the left contact and winding of chain relay 408 to battery. Relay .408 is held in this circuit as long as relay 254 remains energized under the control of relay 406. Relay 408 in operating also closes a circuit for operating the register relay 415 of the first code register 402. This circuit may be traced from the grounded conductor 413, right front contacts of relay 408, conductor 414, through the operated contacts of relay 409 and the left winding of register relay 415 to battery.

Following the completion of these circuit operations the 'slow to-operate relay 323, the circuit of which was closed by relay 406 as previously explained, attracts its armatures and locks in a circuit from battery through its winding and inner left contact to ground at the off-normal springs 334, which were closed when the switch 300 moved oif normal. Relay 323 at its outer left contact closes a circuit for release magnet 335, which energizes and restores the switch 300 to normal. As the switch 300 returns to normal, contacts 334 are opened, and the locking circuit of relay 323 is severed to permit the release of this relay. The operation of relay 323 and the return of wiper 303 to normal open the energizing circuit of the chain relay 406. Relay 406 opens the operating circuit of relay 406 and the operatingcircuit of relay 254. Relay 254 releases, but relay 408 remains locked until the closed contacts of relay 254 are opened. However, before the closed contacts of relay 254 are opened on the release of this relay, the normal contacts of the relay are again closed,and a circuit is completed from ground over the contacts of relay 255, nor- 'mal contacts of relay 254, conducto'r 417, through the normal contacts of relays 411 and 41 0 in series, winding of relay 4:16, right eoirtacttand 7'5 right winding of the operated register relay 415 to battery. Relay 413 operates in this circuit and closes a locking circuit for itself and the operated register relay 415 to ground over control conductor 418 before the chain relay 408 has time to release following the severance of its locking circuit when the front contacts of relay 254 are finally opened upon the release of the latter relay. Relay 436 in operating releases relay 409 to disconnect the chain relays 401 from the register 402. Relay 436 also extends the grounded conductor 418 to the winding of relay 410 by way of conductor 421. Relay 410 thereupon operates and associates the chain relays 401 with the next code register 403. When relay 323 operates, as above described, in response to the operation of chain'relay 406, it connects conductor 251 through its middle le ft contact to conductor 252, thereby completing the circuit for the transmission of current of the frequency f3 from the source 212 over the subscribers line to energize the escapement magnet 157. A moment later, when relay 323 releases in response to the release of the switch 300, the source 212 is disconnected from the line, magnet 151 deenergizes, and the wipers 131 and 132 advance to their third position contacts 1 and 166.

The subscribers transmitter is now in condition to transmit the second code character H to the central oflice. With wipers 131 and 132 standing on terminals and 166, the wipers 1 12 and 1 13 of the transmitter are effective to include the corresponding condensers in the network, resuiting in a phase shift of the harmonic by an amount corresponding to the character H or its numerical equivalent 4. This harmonic flows overthe subscribers line to the transformer 352 and produces an unbalance corresponding to the numeral 4. Relay 306 operates, and relay 30'! releases to render the relaxation oscillator 31! effective. The 0sci1lator'31'l steps the switch 330 from one position to another until a balance is attained, at which time relay 301 is operated to discontinue the stepping movement. Since the digit being transmitted is 4, the wiper 303 is now standing on its fourth position terminal 336, and relay 30'1 in operating extends ground over conductors 328, 329 and 330 through the wiper 303 and terminal 336 to conductor 33'! and thence through the winding-of chain relay 423 to battery. Relay 42 3- operates the associated chain relay 424 and also closes a circuit, previously described, for operating relay 254. Relay 424 extends the grounded conductor 413 through its right contact over conductor 425 through closed contacts of relay 410 and the left winding of register relay 426 to battery. When relay 323 subsequently operates, it releases relay 423, in turn releasing relay 254, resulting in the locking of register relay 426 and relay 422 to ground over the control conductor421. Also relay 323 causes the release of switch 300 and causes the application of current from source 212 to the subscribers line to energize the magnet 157 at the substation. Finally when relay 323 releases, the magnet 151 deenergizes and the escapement switch 129 advances its wipers to the fourth position terminals. Also *the relay 422 in operating releases relay 419 todisconnect the register 403 and closes an obvious circuit for operating relay 411 to associate the next code register 404 with the chain relays 01 The escapement switch 129 standing in its fourth position now associates the wipers 114 and 1-15 with the transmission circuit, and the phase of the harmonic is is shifted by an amount corresponding to the third code letter E or its numerical equivalent 3. --At the central office the tubes 304 and 305 are unbalanced, and the switch 300 steps until the wiper 303 reaches its third position terminals 338. The circuit is then balanced, the switch ceases stepping, and the grounded circuit previously described is extended over wiper 303 and conductor 339 to operate the chain relay 428. Relay 428 in turn operates relay 429 and relay 254 as above described. Relay 429 applies ground from conductor M3 and its right contacts over conductors 430 and 4M through the operated contacts of relay 4II to operate register relays 43I and 432. Following the operation of the register relays, relay 323 operates to bring about the release of switch 300 and to transmit another impulse of current from the source 212 to energize the escapement magnet I51 at the substation. Thereafter relay 323 releases causing the release of the chain relays 428 and 429, the locking of the register relays 43I and 432 through their right windings and the winding of relay 433 to the grounded control conductor 434. Relay 433 deenergizes relay 4II to dissociate the register 404 from the chain relays 401. Relay 323 in releasing also removes the impulse of frequency is from the subscribers line, and magnet I51 releases to escape the wipers I31 and I32 to their fifth position terminals IE1 and IE5.

It will be noted that the fifth position terminals of the escapernent switch I29 lie intermediate the three positions relating to the three office code characters C, H and E and the succeeding four positions which relate to the four numerical characters 5, 6, '7, and 8 of the wanted designation. When the wipers I3I' and I32 reach these intermediate terminals I61 and 168,-respectively, the phase-shifting networks associated with the subscribers transmitter are removed from the transmission circuit, and the phase of the harmonic f2 now transmitted over the subscribers line to the central office is influenced only by the characteristics of the line its-elf. The circuit for the transmission of the harmonic f2 from the substation, which is now free from the phase-shifting networks, may be traced from the lower left terminal of filter I28, conductor I53, wiper i3I, terminal I81, conductors I52 and I5I through the lower coil of transformer I44, conductor I50, wiper I32, terminal I88, conductors I49 and I48 to the other terminal of the filter I28. The two harmonics 12 now produce a balanced condition of the circuit at the central office, and relay 301 is the relaxation oscillator 3I1. Moreover, the oscillator is further disabled by the operation of any one of the relays of register 404, severing the discharge circuit of the tube 3I'8.

The originating register sender having thus received the office code registration now proceeds in the well-known manner to utilize this registered information for the purpose of controlling the selective operation of the district and office switches S-I and S-2 to seize an idle trunk 208 extending to the terminating equipment in the ofiice containing the called subscribers line. While the sender is controlling the selective movement of the switches the-subscribers designation transmitter remains quiescent. As soon as the trunk 200 is seized, the sender selector switch 3-6 operates to find an idle one of the terminating register senders, such as the one shown in Figs. 5, 5 and 1. Having held operated to disable thus controlled the extension of the connection through to'the terminating sender the originating sender may now be released. The specific manner in which the originating register sender proceeds through its releasing operation and the manner in whichthe link circuit interconnecting the finder switch F and the district selector switch SI is completed through to the selected trunk 200 are not shown in detail herein since these releasing and circuit controlling functions are well known to those skilled in the art. It is suificient to note that following the control of the switches S-I and S-2 by theoriginating sender the operated register relays are released by the removal of ground potential from conductors M8, 421 and 434, the energized relays 2I3, 222, 22I, 209 and 2I0 are released, the subscribers line is disconnected from the originating sender, and the contacts of sequence switch springs 2I5, 2I8, 2'51 and the subscribers line through the brushes of switches S-I and S-2 to the trunk 200.

Upon the extension of the subscribers line through to the terminating sender, shown in Figs. 5, 6, and '7, a circuit is closed from battery through the winding of relay 508, conductor 509, brush 5H), contacts of spring 5 of the sequence switch associated with the incoming selector switch S-4, thence over the lower side of the trunk200 to the subscribers line I00 and as previously traced through the winding of transformer I44, and returning over the other side of the subscribers line :to the upper conductor of trunk 200, through the contacts of sequence switch spring 5I2, .brush 5I3, conductor 5I4 through the winding of reactance coil 5I5 to ground. Relay 508 operates and closes obvious operating circuits for relays 5I'6 and 5I1. Current of fundamental frequency i1 is now supplied from the terminating sender over the calling subscribers line, the same as it was from the originating sender, to energize the harmonic generator I21 at the substation for the purpose of producing thereat the harmonic frequency f2. The circuit for supplying frequency 11 may be traced from the right terminal of source 500, conductor 5I8 through the filter '5I9, thence over conductor 5I4 and the upper conductor of trunk 200 and line I00, through the transformer I44 as previously traced, and returning over the lower side of the connection through conductor 509 thence over conductors 520 and 52I, through the filter 5I9 and conductor 522 to the other pole of the source 500. Current from the fundamental source 500 is also applied to the harmonic generator 50! to produce a local harmonic current of frequency 12 for purposes of comparison. Harmonic jz is selected by filter 523 and is applied by way of phase shifter 502 and the normal contacts of relay 524 to the primary windingof transformer 525 in the input circuit of the balancing tubes 503 and '504. At the same time the harmonic f2 produced by the subscribers station is returned over the line I00 and the trunk 200 to the terminating sender where it is selected by the filter 528 and applied by way of amplifier 521 and the normal contacts of relay 524 to the transformer 528 also associated with the input circuit of the balancing tubes 503 and 504.

It will be noted that the connection extended to the terminating sender now includes the trunk line200 in addition to the subscribers line I00. This additional transmission circuit produces its effect upon the phase of the harmonic f2 flowing from the subscribers station, and it is now necessary to make an adjustmentv in the phase of the 258 are closed to extend,

l2,257,5es

harmonic f2 produced locally in the terminating sender to compensate for the phase displacement produced by the entire connection, including the line I and the trunk 200, between the subscribers station and the sender. This compensating adjustment, which occurs While the subscribers transmitter is pausing between the office code and the numerical designation, is performed by the balancing circuit including the tubes 503 and 504 and the phase shifter 502 in the same manner as the compensating adjustment previously described in connection with the originating sender. Normally, that is, before the sender is seized, the tubes 503 and 504, which operate on the non-linear part of their characteristics, are in a balanced condition, and polarized relay 529 is standing with its armature in the mid-position. The control gaps of the gasfilled tubes 505 and 505 are, therefore, ionized. But the relays 530 and 53I do not operate since the anode circuits of these tubes are open at the outer right contacts of relay 5I6. Relay 5I6, being slow to. operate, does not close the anode circuits of the tubes 505 and 506, following the seizure of the sender as above explained, until suffic-ient time has elapsed to allow the application of the local and incoming harmonics to the input circuit of the balancing tubes 503 and 504. When these harmonics are applied to the balancing tubes, an unbalance is produced, and relay 529 operates to one or the other of its contacts depending upon the sense of the unbalance. Accordingly, th relay 529 places a short circuit around the control electrodes of one of the gasfilled tubes 505 or 506, quenching the discharge of that particular tube. As soon thereafter as slow relay 5") attracts its armature, the anode circuits, including battery 532, are closed, and discharge occurs across the main gap of the tube whose control gap remains ionized. This results in the operation either of relay 530 or 53I as the case may be, causing in turn the operation of relay 533 or 534 to close the armature circuit 530 of the phase-shifting motor. The circuit for the field 535 of the motor is closed by slow-release relay 531 which also operates when relay 5I6 attracts its armature.

Thus the phase shifter is driven to produce a continuous change in the phase of the local harmonic applied to the input circuit of the tubes 503 and 504. When the phases of the two harmonies are brought into quadrature relation, a balanced condition results, and relay 529 returns its armature to the mid-position. This permits the other gas-filled tube to ionize whereupon the other one of the two relays 530 and 53I operates. Since both relays 530 and 53I are now operated, the operated one of relays 533, 534 releases, and the energizing circuit of the motor armature is-replaced by a short circuit, thus bringing the phase shifter 502 to rest. A moment later relay 531;,releases and opens the field circuit. Relay 524 also operates, extending the circuit for the local harmonic and the circuit for the incoming harmonic to the-input circuit of the balancing tubes EDI and 602. Relay 524 in operating transmits an impulse of current of frequency is from the source 501 back to the calling subscribers line.- The circuitmay be traced from the lower terminal of source 501, normal contact of relay 603, front contacts of relay 524, conductor 5I4, thence over the connection through the transformer I44 at the calling station and returning to conductor 509 and the other terminal of source 501. 'Current 'from the source 501 is rectified by the rectifier I56 and energizes the escapement magnet I51 to prepare for the advance of wipers I3I and I32 to the next terminals in the bank.

The moment when slow-to-operate relay 603 operates, its circuit having been closed by relay 524, the impulse from source 501 is discontinued, and magnet I51 at the substation releases to move the wipers I3I and I32 to their sixth position terminals I69 and I10, respectively. In this position the phase-shifting network corresponding to the thousands digit 5 is included in the transmission circuit, and the phase of the harmonic is now transmitted back to the terminating sender in the central ofiice where it unbalances the tubes GUI and E02, permitting the differential relay 504 to operate and release relay 605 in case the latter relay is energized at this time. In a manner similar to that already described in conncction'with the originating sender, the relaxation oscillator 606 is rendered effective to operate the stepping magnet 601 to advance the switch 600 from position to position, including resistances Hi3 and 6H and successive condensers in shunt relation with the reactance coils E08 and 609, until the phase of the local harmonic i2 is brought into quadrature relation with the phase of the incoming harmonic. When this occurs, relay 605 operates, and a circuit extended over wiper 6I0' operates the relay 105 in the chain 100. The operated relay 105 in the chain operates the co-rresponding'relay 106 in chain MI and also operates relay 538. Relay 106 causes the operation of register relay 102' of the thousands register 102, relay 108 having previously operated at the time the sender was seized. Relay 101 looks in series with relay 109 when relay 538 releases in response to the release of relay 105, which occurs as soon as the slow-to-operate relay BII subsequently operates, the circuit of the latter relay having been closed by the operation of chain relay 105. Relay 109 operates, releasing relay 108 and operating the next connecting relay 1I0 to associate the next or hundreds register 103 with the chain relays. When relay 6II operates, it causes the release of switch 600 and prepares the balancing circuit for receiving the next digit.

In a similar manner the next three digits of the number, the hundreds, tens and units digits, are transmitted from the calling subscribers station to the incoming sender and registered on the relays of the corresponding registers. At the end of each balancing operation relay Ell operates to send an impulse from the source 501 to energize the escapement magnet I51 and upon the return of switch 600 to its normal position relay 6II releases to terminate the impulses. After all numerical digits hav been received and registered on the registers 102, 103 and 104, the terminating sender proceeds in the well-known manner to control the selective operation of the incoming and final selector switches S4 and S5 to complete the extension of the connec tion through to the called subscribers line 501.

- Thereupon the incoming sender, including all operated relays therein, is restored to normal condition, and the talking connection is placed under the control of the subscribers.

At the calling subscribers station it will be noted that the escapement switch I29 returns to its normal position (indicated in the drawing) following the transmission of the final or units digit. The subscriber, observing the return of the switch I29 to its normal position, by any suitable indicating means, such as an index pointer on the handle I40, knows that the number has been fully transmitted and that he may now disconnect the transmitter from the line. To do this he restores the key I43 to its normal condition. The restoration of the key I43 disconnects the transmitter and substitutes for the primary winding of the transformer I44 the bridge through the substation instrument. The continuity contacts of the key I43 make this substitution possible without opening the loop of the line I to affect the holding condition at thecentral office. The subscribers may now converse with each other.

When conversation is over, the subscribers release the established connection in the usual manner by replacing their receivers on the switchhooks.

Having considered this system in its details, it will be seen that it has numerous advantages over the systems of the prior art. 'It is possible to transmit an entire designation over a sub scribers line to a central offic by using current of a single frequency, making it unnecessary to provide expensive filtering equipment at the central office; it is possible for the subscriber to set up the designation in advance on his transmitter where it remains as a record of the call that he is making and also a record of the call until the next time he wishes to use his transmitter; and it is possible to divide th designation into its code and numerical parts and to transmit each one separately to the central ofiice, making the necessary adjustments for the line characteristics.

What is claimed is:

1. The combination in a communication system of a central ofiice, a subscribers line having a substation thereon and terminating in said central ofiice, a source for producing alternating currents at the central oiiice and for transmitting them over said line to said substation, designation means operable by the subscriber at said substation for'producing predetermined variable shifts in the phase of the current flowing in the line to represent the characters of a wanted designation, means at the central omce for identifying the characters of said designation by determining the amount of phase variation produced at said substation, automatic mechanisms, and means for operating said automatic mechanisms in accordance with the identified characters of said designation.

2. The combination in a telephone system of a central office, a subscribers line terminating in said ofiice, a source at the central office for pro-. ducing alternating currents in said line, subscriber-controlled means at the substation of said line for producing variable shifts in the phase of the current flowing in the line to represent the characters of a wanted designation, means at the central ofiice for determining the amount of phase shift produced in said currents at the substation by comparing the currents of shifted phase with the current produced by said source atthe central oiiice, means at the central office for utilizing these phase comparisons to register the characters of said wanted designation, and automatic switches controlled by said register means for extending said subscribers line.

3. The combination in a telephone system of a central office, a subscribers line terminating in said office, a source of alternating current of fundamental frequency at the central oiiice, means for connecting said source to the line to transmit current thereover, means at the substation of said line for producing a harmonic of said fundamental frequency, means for varying the phase of said harmonic current to represent the characters of a wanted designation and to transmit said harmonic current back to the central ofiice, means at the central ofiice for determining the amount of phase variation in said harmonic current to identify the characters of said designation, and means operable in accordance with the identified characters of the designation for establishing a telephone connection.

4. The combination in a telephone system of a central ofiice, a subscribers line terminating therein, a source of current of fundamental frequency at the central office, means for transmitting said current over the line to the substation thereof, means at the substation for producing a harmonic of said fundamental frequency cur-'- rent and for transmitting it back over the line to the central office, means at the central oflice for producing a harmonic of the same frequency as the one produced at the substation, means for varying the phase of the harmonic produced at th substation in accordance with th digital values of a wanted designation, means at the central office for comparing the phases of the harmonic incoming over the subscribers line with the phase of the harmonic produced locally in the central office to identify the designation, and means operable in accordance with the identified designation for establishing a telephone connection.

5. The combination in a telephone system of a central ofiice, a subscribers line terminating therein, a source of current of fundamental frequency at the central ofiice, means for transmitting said current over the line to the substation thereof, means at the substation for producing a harmonic of said fundamental frequency and for transmitting it back over the line to the central office, means at the central office for producing a harmonic of the same frequency as the one produced at the substation, means for varying the phase of the harmonic produced at the substation in accordanc with the digital values of a wanted designation, a phase balancing mechanism at the central oflice for comparing the phases of the harmonic incoming over the subscribers line and the harmonic produced 10-- cally in the central ofiice, means to shift the phase of the local harmonic until said mechanism balances, and means controlled by said shifting means for identifying the digital characters of said designation.

6. The combination in a telephone system of a central office, a line terminating in said ofiice, means at the substation of said line for producing current of a given frequency, means at the central ofiice for producing current of said frequency, means including networks selectable by th subscriber for causing variable shifts in the phase of the current produced at the substation to characterize the digits of a telephone designation and for transmitting it over said line to the central ofiice, networks at the central office for varying the phase of the current produced at the central oflice until a definite phase relation with the current incoming over said lin is reached, means at the central oflice for identifying the digits of said designation by measuring the amount of phase shift required at the central office to reach said definite relation, and means controlled in accordance with the designation identified for establishing a telephone connection.

7. The combination in a telephone system of a telephone line having a substation thereon, a designation transmitter at said substation and means for variably presetting it in accordance with the successiv characters of a designation, networks of different values selected by said transmitter in its preset position to represent the successive characters of the designation, a source of current of a given frequency, means for subjecting said current to the selected networks successively to produce corresponding shifts in the phase thereof and for applying said current to said line, and means associated with said line for determining the amount of the successive shifts produced in the current flowing in said line to identify the characters of the designation preset on said transmitter.

8. In combination, a subscribers line having a designation transmitter for sending designat ons comprising a plurality of successive digital characters, means for presetting said transmitter in accordance with an entire designation, means for producing an alternating current in said line, means for causing a partial operation of said preset transmitter to produce successive shifts in the phase of said current to represent a portion of the digital characters of said designation, means for detecting said shifts of phase to identify the particular characters represented, automatic switches controlled in accordance with the digital characters identified for partially extending said line, means for thereupon causing said transmitter to complete its operation to produce further shifts in the phase of the current in said line to represent the remaining characters of said designation, means for detecting said further phase shifts to identify the characters involved, and other automatic switches operated in accordance with the characters so identified fo-nfurther extending said line.

9. The combination in a telephone system of an originating ofiice, a subscribers line terminating therein, a terminating office, a trunk line interconnecting said ofiices, a source in the originating o-fiice for producing alternating currents in said subscribers line, means at the subscribers station for varying the phase of currents flowing in the subscribers line to represent the code and numerical portions of a Wanted designation, means at the originating ofiice for measuring the phase variations representing the code part of the designation to cause the registration in the originating office of said code, automatic switches in the originating ofiice con- .trolled by the registered code designation for extending said subscribers line over said trunk to the terminating ofiice, a source in the terminating office for producing alternating currents in the extended connection including said subscribers line, means in the terminating office for measuring the phase variations produced in said subscribers line representing the numerical part of said designation and for causing the registration of said numerical designation in said terminating office, and automatic switches in said terminating oflice controlled by said numerical registration for further extending said connection. I

10. In a telephone system, a central ofiice, a subscribers line terminating therein, means at the substation of said line for producing a current of a given frequency and for transmitting it over said line to the central o-ffice, means at the central oflice for producing a current of the same frequency, means at said substation for varying the phase of said first current to characterize a designation, means at the central ofllce for comparing the phases of said first and second currents to identify said designation, and means for producing an adjustment in the phase of said current produced at the central oflice to compensate for the shift produced by said line in the phase of the current flowing thereover from the substation.

11. In a telephone system, a central ofiice, a subscriber's line terminating in said ofiice, means for producing a first current of a given frequency and for transmitting it from the substation over said line to the central oflice, means in the central ofi'ice for producing current of the same frequency, means for adjusting the phase of the central oflice current to compensate for the shift produced by said line in the phase of said first current, means at the substation for varying the phase of said first current to characterize a designation, means for comparing the phases of said first current and said central office current to identify a portion of said designation, automatic switches controlled in accord anc with the identified portions of the designation for extending a connection from said line, means for adjusting the phase of the central ofiice current to compensate for the shift produced by said extended connection in the phase of said first current, means for comparing the phases of said first current and said central oflice current to identify the remainder of said designation, and other automatic switches controlled in accordance with the remainder of the identified designation for further extending said connection.

12. The combination in a telephone system of a central ofiice, a telephone line terminating in said ofiice, a station on said line, a source of alternating current at the central office, means, for connecting said source to the line to transmit current thereover, means at said station responsive to the current in said line for producing alternating current of a particular frequency diifering from the frequency of said source, means for varying the phase of the current of said particular frequency to represent the characters of a telephone designation and to transmit the same back to the central ofiice, means at the central ofiice for determining the amount of phase variation in the current of said particular frequency to identify the characters of said designation, and means operable in accordance with the identified characters for establishing a telephone connection.

ROBERT C. MATHES. 

